Newer designs and manufacturing techniques have driven electronic components to small dimensions and miniaturized many communication devices and systems. Unfortunately, antennas have not been reduced in size at a comparative level and often are one of the larger components used in a smaller communications device.
Circular polarization is often used in systems for communicating with earth orbiting satellites and airborne vehicles. Circularly polarized systems are advantageous in these applications because they are resistant to multipath effects, and resist the effects of fading caused by mismatched polarizations due to aircraft pitch and roll.
For background, an antenna is a transducer that converts radio frequency electric current to electromagnetic waves that are then radiated into space. The antenna may also connect electromagnetic waves into electric current. The electric field or “E” plane determines the polarization or orientation of the radio wave. In general case, most antennas radiate either linear or circular polarization.
A linearly polarized antenna radiates in one plane. In a circularly polarized antenna, the plane of polarization rotates in a circle making one complete revolution during one period of the wave. If the rotation is clockwise in the direction of propagation, the sense is called right-hand-circular polarization (RHCP). If the rotation is counterclockwise, the sense is called left-hand-circular polarization (LHCP).
An antenna is said to be vertically polarized (linear) when its electric field is perpendicular to the Earth's surface. An example of a vertical antenna is a broadcast tower for AM radio or the “whip” antenna on an automobile. Horizontally polarized (linear) antennas have their electric field parallel to the Earth's surface. Television transmissions in the United States typically use horizontal polarization.
A rotational polarized wave radiates energy in both the horizontal and vertical planes and all planes in between. The difference, if any, between the maximum and the minimum peaks as the antenna is rotated through all angles, is called the axial ratio or ellipticity and is usually specified in decibels (dB). If the axial ratio is near 0 dB, the antenna is said to be circularly polarized. If the axial ratio is greater than 1–2 dB, the polarization is often referred to as elliptical.
Circular polarization is most often used in satellite communications. This is particularly desired since the polarization of a linearly polarized radio wave may be rotated as the signal passes through any anomalies (e.g. Faraday rotation) in the ionosphere. Furthermore, due to the position of the Earth with respect to the satellite, geometric differences may vary especially if the satellite appears to move with respect to the fixed Earth bound station. Circular polarization will keep the signal constant regardless of these anomalies. Circularly polarized antennas are normally more costly than linearly polarized types since true circular polarization is difficult to attain. An example of a true circularly polarized antenna is the helix.
Quadrifilar helix antennas (QHAs) are known in the art to be circularly polarized while providing positive gain for any visible satellite location. The basic design of a QHA, such as that disclosed in U.S. Pat. No. 6,812,906 to Goldstein et al., includes two bifilar helical loops, each having two legs. These loops are oriented in a mutual orthogonal relationship on a common axis. Each of the four legs of this antenna is fed a signal that is 90 degrees apart in phase (i.e., in phase quadrature).
Other types of common circularly polarized antennas include dipole turnstiles, crossed slots and monofilar helix antennas such as a Wheeler Coil. However, an enhanced design for a small circularly polarized antenna with a hemispherical pattern may be desired for high frequency (HF) applications, low earth orbit satellites, airborne, and other mobile communication systems, for example.